In lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful as lithographic printing plate precursors typically comprise an imageable layer applied over the hydrophilic surface of a substrate. The imageable layer includes one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material.
Conventionally, imaging of the imageable element with ultraviolet and/or visible radiation has been carried out through a mask, which has clear and opaque regions. However, direct digital imaging, which obviates the need for imaging through a mask, is becoming increasingly important in the printing industry. Thermally imageable elements that are to be imaged with infrared radiation, such as with a laser or an array of lasers emitting modulated near infrared or infrared radiation in the range of about 800 nm to about 1200 nm typically about 830 nm, about 1056 nm, or about 1064 nm, typically comprise an infrared absorber known as a photothermal conversion material. Photothermal conversion materials absorb radiation and convert it to heat.
These imageable elements typically require processing in a developer to convert them to lithographic printing plates. Developers are typically aqueous alkaline solutions, which may also contain substantial amounts of organic solvents. Because of their high pH and the presence of organic solvents, disposal of substantial quantities of developer is expensive and can cause environmental problems. Processing of the imaged imageable element in a developer also introduces additional costs in, for example, the cost of the developer, the cost of the processing equipment, and the cost of operating the process.
On-press developable lithographic printing plate precursors can be directly mounted on a press after imaging and developed with ink and/or fountain solution during the initial press operation. These precursors do not require a separate development step before mounting on press. On press imaging, in which the precursor is both imaged and developed on press, eliminates mounting the precursor in a separate imaging device. On-press developable thermally imageable elements require compounds that absorb infrared radiation and are soluble in, or removable by, fountain solution.
Thus, a need exists for infrared absorbers for imageable elements that do not require developers that have a high pH and/or contain organic solvent. Preferably, these infrared absorbers are soluble in or removable by ink and/or fountain solution so that they may be used in on press developable imageable elements.